The previous rules apply to the initiator of a shidduch, employment or business partnership. Halachah classifies such a person as an advisor and requires that he not make the suggestion if he has reason to suspect that it may not be in the best interest of either party.
A similar set of rules applies to one who did not make the suggestion, but whose advice is sought by one of the parties. He too must not be guilty of misleading the people involved. In fact, his responsibility is even greater than that of the shadchan (matchmaker). Whereas the shadchan merely suggests that the shidduch be considered, the advisor often states a definitive opinion regarding the proposed relationship which may profoundly influence the party’s final decision.
Giving advice is not a matter to be taken lightly. One does not give guidance unless he believes he understands the situation, and has the insight and life experience necessary to properly direct his petitioner. One should never encourage a match he knows nothing about for the sake of seeing to it that the person “finally gets married”. One may encourage a match only if he sincerely believes that it is good for both parties.
Thus, while the advisor’s first obligation is to the party he is advising, it is forbidden for him to encourage a shidduch that he clearly knows is bad for the other party. This would be considered speaking loshon hora in the form of speech that causes harm.
It is forbidden for an advisor to discourage a relationship unless his disapproval is based on firsthand information that was carefully analyzed.
Finally, if an advisor encouraged his petitioner to pursue a given shidduch but his advice was ignored, he may not draw the conclusion that the person “just doesn’t want to get married,” is “incapable of making commitments.,” or has “unrealistic expectations.” To make such statements would be to speak loshon hora and perhaps hotzaas shem ra (slander).
Sefer Chofetz Chaim, 26 Kislev, Page 220 and 409